VEGGIE-SAURS OF HALF CHICKEN SIZE: ENIGMA OF NANOSAURUS & DRINKER AND OTHER TINY HERBIVORES AT COMO BLUFF

Famed for huge “veggie-saurs”, Como Bluff, Wyoming, and other sites in the Morrison Formation, offer a puzzling history of small and mid-sized bipedal herbivores. The most complex dentition occurs in “Curved-Tooths”, camptosaurs and dryosaurs, where stacks of replacement teeth are curved so more crown volume is packed into jaws. The “Straight-Toothed” herbivores have simple vertical tooth replacement, seemingly less effective for tough plant food. The smallest named Straight Tooth is Nanosaurus agilis, "Half a Domestic Fowl” in size according to 1877 reports. The type lower jaw, from Colorado, is, in our opinion, strangely lizard-like and unique: tooth roots are widely exposed, resembling pylons; crown tips are aligned. Alas, breakage removed all crown details. Better documented is Othnielosaurus consors: the type from the upper zone of the Lower Morrison at Como is augmented by skeletons elsewhere in the Formation. Body bulk is that of a Brocket Deer. Eruption of teeth is staggered so crowns do not align; adjacent teeth overlap; ridges fan out from crown bases to marginal cusps; dental geometry is more complex than in ground iguanas (Conolophus), suggesting medium-tough food. Specimens occur mostly as singletons in channels and floodplain paleo-soils; on average, the species is greatly outnumbered by Curved-Tooth herbivores.

The uppermost facies in the Como Morrison is a dark smectitic mudstone interrupted by beds of silicified mudstone. Here a Straight Tooth, Drinker nisti outnumbers all other dinosaur species combined, five-to-one. Adult size is medium-small, 2/3 of O. consors (femur L 160 v 250 mm). Dentition follows O. consors but with heavier tooth-to-tooth wear and an enlarged lower outer basal cusp. The Drinker head is relatively enlarged so the ratio of tooth volume to body mass is increased. Three dozen partial skeletons from four single-species aggregates, where flexed limbs are crowded together, suggest communal burrows or lairs. Adults, half-grown and small juveniles are packed side by side. How could this one species of Straight Tooth achieve numerical dominance? Varied and common fish, superabundant shed teeth from “crocodiles” (Diplosaurus) and pieces of aquatic turtle indicate wide lakes and swamps. Perhaps Drinker’s spreading hind feet, complex occlusion and colonial foraging opened patches of hydrophilic vegetation for exploitation.